Mechanism of Earthquake Forces & Methods of Resisting Seismic and Wind Forces

Drift:

Drift is defined as the difference in lateral deflection between two adjacent stories. Lateral deflection is the predicted movement of a structure under lateral loads. During an earthquake, large lateral forces can be imposed on structures; both the 1997 UBC (the basis of the 2001 California Building Code) and ASCE 7-02 (which is based on NEHRP) require that the designer assess the effects of this deformation on both structural and non-structural elements. Lateral deflection and drift have three primary effects on a proper consideration during the design process, large deflections and drifts can have adverse effects on structural elements, nonstructural elements, and adjacent structures.

Figure-01 indicates that how a building can be affected by drift in an earthquake. The movement can affect the structural elements (such as beams and columns); the movement can affect the non-structural elements (such as windows and cladding); the movements can affect adjacent structures.

Figure-01:  Building affected by Drift in an Earthquake Force

Effect of Drift on a Structure:

In terms of seismic design, lateral deflection and drift can affect both the structural elements that are part of the lateral force resisting system and structural elements that are not part of the lateral force resisting system. In terms of the lateral force resisting system, when the lateral forces are placed on the structure, the structure responds and moves due to those forces. Consequently, there is a relationship between the lateral force resisting system and its movement under lateral loads; this relationship can be analyzed by hand or by computer. Using the results of this analysis, estimates of other design criteria, such as rotations of joints in eccentric braced frames and rotations of joints in special moment resisting frames can be obtained. Similarly, the lateral analysis can also be used and should be used to estimate the effect of lateral movements on structural elements that are not part of the lateral force resisting system, such as beams and columns that are not explicitly considered as being part of the lateral force resisting system.

Design provisions for moment frame and eccentric braced frame structures have requirements to ensure the ability of the structure to sustain inelastic rotations resulting from deformation and drift. Without proper consideration of the expected movement of the structure, the lateral force resisting system might experience premature failure and a corresponding loss of strength. In addition, if the lateral deflections of any structure become too large, P-Δ effects can cause instability of the structure and potentially result in collapse. Structural elements and connections not part of the lateral force resisting system need to be detailed to withstand the expected maximum deflections and drifts. Though these elements are generally ignored during the design lateral analysis, they must effectively “go along for the ride” during an earthquake, meaning that they experience deflections and rotations similar to those of the lateral force resisting system. Consequently, both the 1997 UBC and ASCE 7-02 require that the structural elements not part of the lateral force resisting system be designed to maintain support of design dead and live loads under the expected deformations, including any PΔ effects. One of the best examples of failure to ensure adequate deformation compatibility was the collapse of the parking garage at Cal State Northridge during the 1994 earthquake. The structure had ductile precast moment frame columns but lacked adequate deformation compatibility in the structural elements and connections that were not part of the lateral force resisting system, resulting in collapse of the interior gravity support system (EERI, 1994).

Mechanism of Earthquake Forces:

As a buildings responds to ground motions produced by an earthquake, the bottom of the structure moves immediately but the upper portions do not because of their mass and inertia. The following figures shows the base of a building moving while the upper part lags behind.

The horizontal force, or the base shear, created by ground motion resulting from an earthquake must be resisted by the building. The more the ground moves, or the greater the weight of the building, the more force must be resisted by the building. When an architect or engineers designs a building, he or she must determine the maximum force a building might have to resist in the future. Buildings are always designed to handle normal vertical and lateral forces.

Figure-02 indicates the mechanism of earthquake forces on a building Structures,

Drawing by Engr. Snehashish Bhattacharjee (Tushar), seasoft022.blogspot.com

Figure-02: Mechanism of Earthquake Forces on a Building Structures

However, once you introduce the possibility of an earthquake, a building must be designed for extraordinary horizontal or lateral forces. The horizontal or lateral forces associated with an earthquake can be thought of as a lateral force applied to each floor and to the roof of a building.

Figure-03 indicates the vertical and horizontal forces on a building during an earthquake,

Figure:a) indicates the direction of the gravitational forces on a building, Figure:b) indicates the horizontal force of seismic waves, and Figure:c) indicates the combined forces of gravity and an earthquake applied to the floors and roof of the building,

Drawing by Engr. Snehashish Bhattacharjee (Tushar), seasoft022.blogspot.com

Figure-03: Vertical and Horizontal Forces on a building during an Earthquake

Diaphragms:

The floor and roof systems that distribute an earthquake’s lateral forces are known as diaphragms. Diaphragms support the gravitational and lateral forces on a building and transfer them to vertical structural elements like braced frames, moment-resistant frames, shear walls. These vertical element help resist lateral forces and are therefore called horizontal or lateral bracing systems.

Figure-04 indicates a diaphragm system between walls,

Figure-04: Diaphragms System between walls

Shear Wall System:

Walls within a building that are designed to receive horizontal forces parallel to the wall are called shear walls. Houses with many rooms separated by structural walls with minimal openings are good examples of shear wall buildings.

Figure-05 indicates shear wall system,

Drawing by Engr. Snehashish Bhattacharjee (Tushar), seasoft022.blogspot.com

Figure-05: Shear Wall System

Braced Frame System:

Braced frame systems are more flexible then shear wall.

In the Figure-06, the dotted lines show the normal position of a shear wall and a braced frame. The braced frame is more flexible and bends farther from its normal position then the shear wall,

Drawing by Engr. Snehashish Bhattacharjee (Tushar), seasoft022.blogspot.com

Figure-06: Normal position of a Shear Wall & Braced Frame System

Moment Resistant System:

This system helps a building resist horizontal or lateral forces at the joints between the columns and beams. These joints become highly stressed, so that they must be constructed of a strong, ductile material like steel. 

Figure-07 indicates a moment-resistant system,

Drawing by Engr. Snehashish Bhattacharjee (Tushar), seasoft022.blogspot.com

Figure-07: Moment-Resisting System